Laser shock strengthening is a new surface deformation strengthening technology. It utilizes stress strengthening and microstructural strengthening induced by high-energy and short pulse laser to realize the surface modification of materials. Compared with the traditional surface strengthening technology, it has distinct characteristics of high voltage (GPa), fast process (ns), high strain rate (107 s−1) and so on, and has significant technical advantages in the controllable strengthening treatment of stress concentration areas of key thin-walled structures in aerospace.
However, process parameters such as laser energy, laser spot diameter and so on, need to be adjusted according to the type and mechanical properties of materials to change the effect of laser shock strengthening. Due to the fact that a reduction of the spot diameter will bring about a reduction in the surface quality, machining efficiency and an increase in the processing cost; at present, the widespread used method in the engineering field to achieve the laser shock strengthening of high strength materials (high strength stainless steel, titanium alloy, nickel base alloy, etc.) is to increase the laser energy, which is inevitable to make the laser energy up to tens of joules. However, at present, due to the limitations of factors such as light frequency, control accuracy, operation cost and volume size, the high power lasers cannot be practically applied in laser shock strengthening field. Therefore, it is a key technical problem in the field of laser shock strengthening to enhance the pressure of laser induced shock wave pressure on the basis of the existing laser energy level.
To solve this problem, CN201210001123, CN200510094810 and CN201310304179 respectively use the high pressure gas, a new water confinement layer and polymer materials to replace the conventional water screen as a new confinement layer to improve the laser induced shock wave pressure. Although these methods make up for some shortage of conventional water screen confinement layer to a certain extent and improve the laser induced shock wave pressure, these methods do not change the generation process of shock wave. Thus, it is impossible to realize the multiple increase of laser shock wave pressure. And the restraint devices of these methods are complex and also cost significantly. The patent application with patent No. CN201310305606 puts forward a pressurized device for laser shock strengthening, which limits the horizontal impact force of the laser shock wave by setting a conical stop sleeve so as to increase the laser induced shock wave pressure. However, this method still cannot realize the multiple increase of laser shock wave pressure. At the same time, it is difficult to achieve multi-point continuous shocking or multi-point overlapping shocking due to that the water film is difficult to keep clean and is easy to splash, and the replacement is complex. In addition, the granted patent with patent No. CN201110120822 presents a method and device for obtaining nano coating on the metal surface by using the strong shock waves generated during the continuous explosion of plasma induced by laser. It significantly increases the laser-induced shock wave pressure by using the continuous explosion of laser in a high pressure resistant glass tube. But the following problems still exist: (1) When the shock waves propagate through the pipe, the pressure decays rapidly, which limit the increase of shock wave pressure generated during the continuous explosion of plasma and makes it difficult to grow exponentially; (2) Shock waves generated by multiple continuous detonation need to be superimposed in multiple times, which requires a very high demand on the control of the explosion time, which further affects the superimposition of shock waves and then reduces the shock wave pressure after multiple error accumulation; (3) The continuous explosion takes a long time, which reduces the processing efficiency.
The granted patent with patent No. CN201210571521 which is closest to the present patent application puts forward a method of carrying out strengthening treatment to the material surface by using a high magnetic field and an electric field to increase the pressure of the laser plasma shock wave. This method has a certain effect on improving the shock wave pressure, but compared to the present application, there are still some deficiencies as follows: (1) Due to the concentration and the charge number of charged particles in the plasma are not uniform, and the motion state changes over time, it is difficult to adjust the size and direction of the magnetic field and the electric field to match them, thus it is difficult to increase the pressure of the shock wave manyfold; (2) In the plasma, since the mass of the charged particles such as electrons are very small and the velocity is very high, it is hard to restrain the charged particles through the magnetic field and the electric field. The reduction of the constraint effect makes it difficult to achieve a multiply increase of the shock wave pressure; (3) The high difficulty in this technology will inevitably lead to complex devices, complicated operation and high cost, so it is difficult to achieve engineering applications.
In order to greatly improve the laser shock wave pressure, and overcome the defects of the existing problems, the present application puts forward a method of using automatic induced photoelectric composite energy field to induce and generate plasma of ultra-high density and ultra-high expansion state, and to exponentially improve laser induced shock wave pressure in a constrained state. The method is simple and easy to implement and the device is of low cost. By searching literatures at home and abroad, there has not found any report relating to automatic induced photoelectric composite energy field at present and using the automatic induced photoelectric composite energy field to induce and generate plasma of ultra-high density and ultra-high expansion state, and thereby improving laser induced shock wave pressure. Therefore, the present invention provides the method and device for the first time.